1. Field of the Invention
The present invention relates to an organic electro-luminescent device, which may hereinafter be referred to as an “organic EL device”, and specifically to an organic EL device that can be easily manufactured by utilizing a specific polyurethane compound, and further, that have improved in brightness, stability and durability.
2. Description of the Related Art
EL devices, which are spontaneous-luminescent all solid state devices, can provide a high visibility and a high impact resistance and therefore are expected to find wide applications. The devices with inorganic fluorescent materials are currently dominant, however, they have problems such as a high manufacturing cost due to the requirement of an AC voltage of 200V or higher for driving, and insufficient brightness.
The study of the EL devices with organic compounds began by using single-crystal materials such as anthracene for thestarter, however, such single-crystal materials gave a thick film of about 1 mm and required a driving voltage of 100 V or higher. Thus, a vapor deposition technique has been tested to form a thin film (Thin Solid Films, Vol. 94, 171 (1982)). The film formed by this method, however, still requires a high driving voltage of 30 V. In addition, the film has a low density of the electron/hole carrier and provides a low probability of photon generation by carrier recombination, giving insufficient brightness.
In recent years, there has been reported an EL device, among EL devices of the function separation type in which a thin film of a hole transporting low-molecular weight organic compound and a thin film of a fluorescent low-molecular weight organic compound having the electron transporting function are laminated sequentially by vacuum vapor deposition method, which can provide high brightness of 1000 cd/m2 at a low voltage of about 10 V (Appl. Phys. Lett., vol. 51, 913(1987)) and, ever since, active research and development has been conducted on layered EL devices.
Such type of EL device, however, can easily form a pinhole since a thin film of 0.1 μm or less is formed through plural vapor deposition steps, therefore, it is necessary to regulate the film thickness under the strictly controlled conditions in order to obtain sufficient performance. Therefore, the productivity is low and the enlargement of area is difficult. In addition, since such type of EL device is driven under a current density as high as several mA/cm2, a great amount of Joule heat generated. In many cases, therefore, the pheromenon wherein the hole transporting low-molecular weight compound and the low-molecular weight fluorescent organic compound each formed into a film in an amorphous-glass state by vapor deposition gradually crystallize and eventually melt so that the brightness is reduced or breakdown occurs. As the result, the problem of life-shortening of the device also occurs. For example, in the case of the electron transporting material, it is proposed so far, as described in Japanese Patent Laid-Open No. 7-109454 (1995), to use oxadiazole derivatives including 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBO). The obtained film, however, can easily crystallize and be also insufficient in terms of charge transportation/injection property. Moreover, because of a poor variety of the electron transporting material other than the oxadiazole compounds, there has currently been a demand for developing a new material excellent in the charge transportation/injection property as well as in terms of low-voltage driving and high efficiency.
On the other hand, with the aim of solving the problems with the productivity and the large-area formation of the layered EL device, research and development has been carried out on EL devices of the monolayer structure. Proposed examples of such devices include a device with a conductive polymer such as poly(p-phenylenevinylene) (Nature, Vol. 357, 477 (1992) etc.) and a device with a mixture of an electron transporting material and a fluorescent pigment in a hole transporting polyvinylcarbazole (the Japan Society of Applied Physics 38th Meeting Proceedings 31p-G-12 (1991)), but they have not yet been comparable in such as brightness and luminous efficiency to the layered EL devices using the low-molecular weight organic compounds.